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About the Staccato controller in a VTTC

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Philcogrump

Tue Jul 11 2006, 03:26AM

Registered Member #212 Joined: Sun Feb 19 2006, 05:42PM
Location: Texas
Posts: 20

I have been looking at many VTTC designs that uses really high-power tubes (beyond 811As) and pretty much all of them use the staccato controller. I understand that it is useful for pulsing the VTTC at a much lower duty cycle to minimize the stress on tubes.

If I recall correctly, wouldn't increasing the grid feedback capacitor also slow down the duty cycle without needing the staccato controller?

The reason I bring this up is because the staccato is solid-state. I figure if a VTTC is ... a VTTC, it shouldn't have any solid-state in it. Steve Ward's staccato outputs really low bps to about 30bps. I don't know if this is wise, but could the triac be replaced with a mechanical relay and use a low power thyratron to drive the relay. A simple tube multivibrator using a dual-triode like 6J6 could be used to produce the low frequency signal to drive the thyratron.

The only shortcoming I can see in this approach is the relay. I figure that a mechanical relay probably can only switch up to about 10bps maximum without great losses. I don't know how a VTTC looks at 10bps or less. Also, there is the possibility of an arc across the relay contacts, or worse, the relay will weld itself shut.

Any comments?

Steve Ward

Tue Jul 11 2006, 04:04AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Provided you use half-wave input to the tank circuit (and not filtered DC), then the relay would have a good amount of time to open and close. You would have to be careful though, as im not sure how "fast" relay's are. You would have to find something that could close maybe in <1mS. Opening is less of a concern, you basically have a full AC half-cycle (8.33mS or 10mS for 50hz) to open it. But, its possible that relays cant even meet this speed requirement.

Running the coil in grid-blocking mode (aka "sputter mode") can increase the chances of tube arc-over from what i hear. Im not sure what the deal is with that, but this is what im told. Sputter mode usually produces an annoying sounding spark, but perhaps you could use a really big cap there and slow things down a lot.

Steve Conner

Tue Jul 11 2006, 10:02AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

You could just use the multivibrator circuit to pulse the grid bias of the main power tube between a couple of hundred volts negative and zero. That way the power tube acts as its own staccato switch.

HV Enthusiast

Tue Jul 11 2006, 11:24AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Posts: 3068

Use a triode tube as a switch and drive with some sort of low frequency oscillator. This can then in turn drive the grid.

Philcogrump

Wed Jul 12 2006, 01:23AM

Registered Member #212 Joined: Sun Feb 19 2006, 05:42PM
Location: Texas
Posts: 20

EVR, are you suggesting to use a triode on the end of the grid feedback coil and drive the grid of that triode with the multivibrator output? This way the grid feedback can be turned on/off and therefore make a staccato-like pulse on the power tube?

HV Enthusiast

Wed Jul 12 2006, 03:20AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Sure why not, but i don't see any reason the controller can't be solid state IMHO. Typically, when any type of equipment is "tube" based, it merely refers to the power component of the device. For example, TWT or magnetron radar transmitters are considered "tube" systems, but still utilize a wide array of solid state components. Even in tube based systems going back 50 years in the 1950's. Same for tube audio amps, tube RF transmitters, etc..., all of which contain a SUBSTANTIAL amount of solid state components.

I would go with the solid state interrupter. It by no means will belittle the title of VTTC.

Even the grand daddy tube audio amplifier of them all, the $350,000 Wavac SH-883 which features a single 833A 150W tube for its main power stage contains a fair amount of solid state components. However, you will still consider that a tube amp, and not a solid state amplifer.

Philcogrump

Wed Jul 12 2006, 03:50AM

Registered Member #212 Joined: Sun Feb 19 2006, 05:42PM
Location: Texas
Posts: 20

I understand your point, EVR. However, I enjoy making devices literally all-tube. This shows how much effort and work was put in just to succeed in building something using simple vacuum tube technology even in areas where solid-state works just fine.

Back to the tube staccato controller for the VTTC...

How would a such triode be oriented in the grid feedback circuit? If the grid needs a negative feedback voltage, then would the cathode of the triode go to the other end of the feedback coil and the plate to ground? If so, then that would be problematic to drive the triode's grid in respect to cathode... unless the tube multivibrator circuit was isolated from the rest so its local ground can be connected to the cathode of the driving triode, and the plate of the triode goes to the high voltage ground.

On the other hand, let say I do not want to use a staccato controller. Instead, I run the VTTC with an undoubled MOT and a power tube that normally could withstand a doubled MOT with a staccato. I assume a such tube could handle the 50% duty cycle at that power level without a problem, right?

Sulaiman

Wed Jul 12 2006, 06:12AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

If you google for "one tube transmitter" you will find a couple of 'keying' techniques
replace the Key with your interruptir relay.

Steve Conner

Wed Jul 12 2006, 09:41AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I agree that Philcogrump should go all-tube for a x5 geek point multiplier.

You're right, the switch tube will need its plate grounded and its cathode connected to the power tube grid circuit, since the grid has to be negative to cut it off. So you'll need a negative HT rail as well as a positive one. I've seen plenty worse in tube circuits, though. >_<

Another possibility would be to use a tetrode as the main tube instead of a triode. Bring the screen supply in through a dropping resistor, and when you don't want the coil to fire, suck the screen supply away by switching it to ground with another tube.

Or, key the cathode current of the main tube the same way as the usual staccato circuit does, but use a thyratron instead of a SCR.

HV Enthusiast

Wed Jul 12 2006, 01:52PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

wrote ...

I agree that Philcogrump should go all-tube for a x5 geek point multiplier.

I sure hope you don't plan on using a solid state diode to rectify the input HV then.

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